The present invention relates to an electric fan device for a vehicle and a method of driving an electric fan device for a vehicle applied to a cooling fan of, for instance, a radiator or a condenser for a vehicle.
Ordinarily, the radiator fan or the condenser fan as the electric fan device mounted on the vehicle is operated or stopped in accordance with the state of the water temperature of the radiator or the pressure of the condenser. An operation is carried out by adjusting a rotating speed stepwise in accordance with a quantity of required cooling.
Usually, the electric fan device for the vehicle of this kind adjusts the rotating speed to three stages including the stop of the radiator fan and the condenser fan, a rotation of 50% and a rotation of 100% by switching a relay to adjust a quantity of cooling to a desired value.
However, when the rotating speed is adjusted by switching the relay, since the adjustment of the rotating speed is limited to the three stages including the stop, the rotation of 50% and the rotation of 100%, a disadvantage arises that a quantity of cooling cannot be properly adjusted to the quantity of required cooling.
Thus, as a device for solving such a disadvantage, an electric fan for a vehicle is provided in which a radiator fan and a condenser fan are driven by using a semiconductor switch element that is PWM controlled. As described above, the radiator fan and the condenser fan are driven by using the semiconductor switch element that is PWM controlled, so that the rotating speed of them can be adjusted without stages.
However, in the above-described electric fan device for the vehicle, not only a circuit is required for treating noise to prevent a transmitting noise generated from the semiconductor switch element during switching a high frequency from being fed back to a battery line, but also heat generated due to a switching loss by switching the high frequency is undesirably increased to enlarge a heat radiating structure.
As a device for solving the above-described disadvantage, Patent Document 1 proposes an electric fan device for a vehicle in which a motor current supplied to a fan motor in the fan is controlled by a semiconductor switch element and the switching operation of the semiconductor switch element is PWM controlled under a frequency of several ten Hz or lower by a control circuit to suppress the generation of transmitting noise during the switching operation and reduce a heat generation due to a switching loss.
Specifically, in the electric fan device for the vehicle disclosed in the Patent Document 1, as shown in FIG. 6, an anode voltage from a battery B is supplied in order to a fan motor in a radiator fan F1 and a part between a source and a drain of a semiconductor switch element T1 through a relay R4 operating in accordance with an on/off of an ignition switch IG, and supplied in order to a fan motor in a condenser fan F2 and a part between a source and a drain of a semiconductor switch element T2 in parallel therewith.
Further, the on/off operations of the semiconductor switch elements T1 and T2 are controlled and the fan motors in the fans F1 and F2 are respectively driven by a fan control circuit 5 in accordance with driving signals from a FAN.TEMP switch 7 and an A/C.PRESS switch 9. At this time, a duty ratio is adjusted in accordance with a detection result detected in a pressure detecting sensor and a water temperature detecting sensor, and the switching operations of the semiconductor switch elements T1 and T2 are PWM controlled by a prescribed low frequency of several tens of Hz or lower.
[Patent Document 1] JP-A-2002-142494
When the operation of the electric fan device for the vehicle disclosed in the Patent Document 1 is considered, below-described things can be understood. Namely, FIG. 7 is a wave form diagram obtained by a measurement when one fan motor is used in the electric fan device for the vehicle disclosed in the Patent Document 1.
In FIG. 7, the frequency of the PWM control by the fan control circuit 5 is set to 12 Hz and the duty ratio is set to 52%. Further, in FIG. 7, a1 designates an output signal of the fan control circuit 5, b1 designates a value obtained by converting an acceleration of a mechanical vibration generated in a bracket for attaching the fan motor to a voltage, and c1 designates a motor current supplied to the fan motor.
It is recognized from FIG. 7 that the mechanical vibration designated by b1 is generated in the bracket for attaching the fan motor synchronously with the rise and the fall of the motor current designated by c1 that is supplied to the fan motor.
FIG. 8 schematically shows the change of the motor current of the fan motor. FIG. 8(a) shows an output from the fan control circuit 5. FIG. 8(b) shows the motor current supplied to the fan motor. Here, the rise time of the motor current supplied to the fan motor is represented by tr and the fall time is represented by tf.
Since the torque of the fan motor is proportional to the motor current, the torque rapidly varies during the rise time tr and the fall time tf. Thus, during the rise time, a distortion is applied to the bracket for supporting the fan motor in an opposite direction to the rotation of the fan motor, so that the distortion to the bracket is released during the fall time.
As described above, during the rise time and the fall time of the current supplied to the fan motor, the distortion and the release of the distortion occur in the bracket for supporting the fan motor, so that the mechanical vibration is generated in the bracket.
Since such a mechanical vibration is transmitted to an interior of the vehicle through a chassis, a problem arise that this vibration gives an unpleased feeling to a driver or the abrasion of various sliding parts is accelerated.